There currently exists a known system for transmitting numerical data appearing in the form of a sequence of words in which the words are successively converted into a variable frequency analog signal whose frequency represents the value of those words. Each period of the signal possesses a period proportional to the value of one word of the sequence to be transmitted. This analog signal is transmitted by any device, in particular by a telephone connection and is then decoded on the receiving location with the aid of a phase locking loop delivering a voltage proportional to the input frequency; this voltage is then converted into a numerical signal so as to restore the original numerical data.
The main defect of this type of system is that each word requires at least one period of the analog signal so as to represent its value. This limitation, which firstly results from the actual principle of the process and secondly the nature of the decoding means used (automatic control of the phase locking loop requiring at least one period), entails for a given frequency band a transmission flow (in words per second) limited to the value of the frequency. In addition, in such a system, it is necessary to introduce between words or groups of words synchronism pulses making it possible to recognize the relative positions of the data according to the decoding principle with the aid of a phase locking loop which generates an uncertainty in the position of the words. In addition, the phase locking loops are used in these systems up to the limit of their possibilities and are extremely delicate to adjust from the industrial point of view.